A crested black macaque has been making news of late for borrowing a photographer's camera and snapping some pics of herself. One of the resulting photos, potentially a nice paycheck for the photographer (probably as much for the origin story as the aesthetic merit) has landed in the public domain because it has no human author.
Granting for the moment that the monkey is not a legal person (Can you take a selfie if you have no self? I'll leave that heady question to higher primates), the question boils down to, can you copyright a photo you didn't "take" or an artwork that you didn't "make"? I put those verbs in quotes because the "making" of a creative work is not as simple as the pressing of a shutter button on a camera. Photographers have been fighting the perception that they "just press a button" for ages.
If I accidentally dropped my camera and it landed on the shutter button and made an exposure, I certainly didn't take that photo with any intention, and gravity has no intention or personhood ether. But the shutter button is just the beginning of the process. I'd have to take the intention to develop and print or download and publish the image, and hardly anybody does any of that without making some choices, such as cropping, exposure compensation, color correction, all manner of retouching, etc. I would argue at the end of that process, I certainly made that photo.
I don't know for sure how much monkeying around David Slater performed on the image file in question, but the example here certainly looks cropped at the very least.
I'd be dismayed if all artwork that contained chance elements, natural processes, accidents, appropriation, or audience participation were all tossed into the public domain with no credit or profit for the artist or creator who orchestrated or even just discovered and published the work.
But if the law falls this way - and copyright law is far from perfect - I would still encourage anybody using this photo for commercial purposes to consider a contribution in lieu of royalties to your local pro-primate organization, perhaps for conservation of their natural habitats or the care of retired professional monkeys (performers, companion and helper animals, etc.) and rescued lab monkeys.